Test documentation (dev setup) on CI

CI currently only tests that documentation can be built after all notebooks are
run and converted from .md to .ipynb format. Running all notebooks takes some
time. This CI step makes sure, that the documentation can also be built without
converting the notebooks first.

Jenkinsfile

@@ -46,6 +46,18 @@ stage("Unit Test") {
         sh("ci/upload_doc.sh ${bucket} ${path}")
       }
     }
+  },
+  // Make sure documentation can be built without building notebooks
+  'Documentation (dev setup)': {
+    node {
+      ws('workspace/gluon-nlp-local-docs') {
+        checkout scm
+        sh """#!/bin/bash
+        cd docs
+        make html
+        """
+      }
+    }
   }
 }